ABSTRACT
Some biological phenomena occur once in a lifetime, whereas others occur repeatedly. Repeated events that occur over regular intervals become cyclic phenomena. One cycle is represented by the necessary route of that phenomenon to repeat itself. The duration of a cycle is defined as a period. The biological cyclic phenomena with a period of 24 hours are termed ‘‘circadian’’ (from latin ‘‘circa diem’’ meaning about one day), those with a period shorter than 24 hours, for instance, the respiratory or the heart beat rhythms, are called ‘‘ultradian,’’ and those with a period longer than circadian, such as the menstrual cycle, are called ‘‘infradian.’’ The rhythmic oscillations of most of these phenomena are under the control of both endogenous and exogenous factors. The former usually correspond to specialized cells that exhibit the property of autodepolarization, called ‘‘pacemakers.’’ The second are represented by all the external factors able to influence or overcome the pacemaker’s rhythm. In animals, the main endogenous clock is constituted by the suprachiasmatic nucleus (SCN) (1), while the dark-light alternation, due to the axial earth rotation, represents the most important external environment (2). Body core temperature, sleep, motor activity, and melatonin (MLT) secretion are typical examples of physiological circadian phenomena. Hypnic headache, paroxysmal nocturnal hemoglobinuria, and restless legs syndrome (RLS) are only a few of many pathological conditions that, for different reasons, follow a circadian rhythm of expression. Chronobiology is a young science that studies the basis of physiological and pathological circadian phenomena. The most common method used by this scientific branch to measure the burden of each exogenous or endogenous factor in driving a specific circadian cycle is to eliminate or strengthen the considered factor, keeping constant all the others, and then detect the consequences on the circadian system. If this operation causes a period shift on the given rhythm, it means that the factor has a role in deciding the circadian oscillation of the system. During the last decades, the application of this study’s paradigm under standard laboratory conditions has noticeably improved the knowledge around the pathophysiology of the circadian phenomena.
